The invention relates to a method of forming a luminescent layer on a glass carrier from a suspension of grains of a luminescent material and a quantity of a binder in a suspension medium, which suspension is contacted with the carrier in such a manner that a layer of the suspension adheres to the carrier, the non-adherent suspension residue is removed and the layer adhered to the carrier is dried. Such a method is known from German Patent Specification 1,184,009.
The invention further relates to low-pressure mercury vapour discharge lamps which comprise a layer applied to a carrier by means of such a method.
Finally, the invention relates to a method of preparing the binder.
The method is used in the manufacture of a luminescent screen, such as is utilized, for example, in cathode-ray tubes and high-pressure- or low-pressure mercury vapour discharge lamps. It is then of major importance that the luminescent layer, which comprises a large quantity of grains of luminescent material, should be very homogeneous and should adhere very strongly to the glass carrier, such as the wall of the discharge vessel of a lamp.
The aforementioned German Patent Specification discloses a method of manufacturing a low-pressure mercury vapour discharge lamp having a tubular discharge vessel whose inner wall is provided with a luminescent layer by means of a suspension comprising luminescent grains and a binder. As suitable binders, there are mentioned in the said Patent Specification inter alia polyethylene oxide dissolved in water and nitrocellulose dissolved in butyl acetate or alcohol. After a layer has been applied, it is dried with the suspension medium evaporating. Subsequently, the discharge vessel is heated, the binder being decomposed and escaping in the form of gaseous products.
The heat treatment is not only a time-consuming additional step in the process of manufacturing the lamp (as a result of which additional means, such as special sintering ovens, are necessary), but is also expensive due to the high energy consumption.
A large number of the binders mentioned in the said German Patent Specification suffer further from the limitation that they are only soluble in particular organic liquids. Especially during the drying process, additional measures are then often required to prevent these solvents (which are often toxic and combustible) from reaching the working space or the environment after evaporation.